The present invention relates to hydrostatic transmissions intended primarily for use in riding lawnmowers, lawn and garden tractors, snow throwers and the like, but may also be applied to larger implements and vehicles.
Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, typically oil, and then back to rotary mechanical motion to rotate a pair of drive axles in order to propel the vehicle. The hydrostatic transmission controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Such transmissions have utilized radial piston pumps and motors, axial piston pumps and motors and hybrid transmissions wherein the pump may be of the radial piston design, for example, and the motor formed as a gear pump. The speed of the output of the transmission is typically controlled by varying the eccentricity of the pump track ring or swash plate angle.
A typical construction of the hydrostatic transmission component of the transaxle includes a pump and motor block, supporting the pump and motor, which requires either partial immersion in transmission fluid, or a conduit to supply the pump and motor block with transmission fluid. Many hydrostatic transmission designs lack a filter to filter foreign particles from the fluid prior to such fluid being introduced to the block. Hydrostatic transmissions operating without a filter may be prone to increased maintenance and a shorter life due to debris, contained in the fluid, damaging bearing surfaces and obstructing fluid passageways. Hydrostatic transmission designs which incorporate a filter generally provide an annular filter cartridge between the block and a lower portion of a casing to filter fluid which is drawn through the periphery of the filter. The axial ends of the filter, otherwise open, are overlaid respectively by the casing wall and the lower surface of the block. The axial end of the filter which abuts the block may not be larger than the surface area of the block or unfiltered fluid will be allowed to pass into the pump and motor block. The filter, generally contained within the surface area of the block, may be prone to premature loading of debris due to its limited size. As the filter becomes loaded with debris, performance of the hydrostatic transmission diminishes and further operation of the transmission may result in permanent damage. However, frequently replacing the filter is not an attractive solution since removing the filter is often difficult, requiring substantial disassembly of the implement which corresponds to a significant expense.
Furthermore, the surfaces of the block and the casing which are in contact with opposing axial filter ends must be relatively smooth and flat to provide a suitable seal to prevent unfiltered fluid to pass into the pump and motor block. One problem with this type of filtering arrangement is that additional machining to the block and/or the casing corresponds to a significant increase in cost.
Yet another type of filter arrangement includes wedging a solid piece of filtering material, having fine pores, between the rough surfaces of the block and the casing to provide filtering of the transmission fluid prior to its introduction to the suction ports of the pump and motor block. A problem associated with the solid filter is that it is prone to rapid clogging or loading, requiring an increase in maintenance, which corresponds to a significant maintenance cost.
The present invention overcomes the disadvantages associated with prior hydrostatic transmissions by providing a hydrostatic transmission assembly including a hydraulic pump and motor mechanism having a pump, a motor and at least one fluid passageway therebetween, a source of transmission fluid, at least one projection attached to the hydrostatic pump and motor mechanism which has a suction port therein, and a filter assembly comprising a filter element attached to an impermeable wall. The pump and motor are fluidly connected through the passageway and the passageway is in fluid communication with the source of transmission fluid through the suction port. The impermeable wall includes at least one hole therethrough and the projection extends through the hole and is sealingly engaged with the impermeable wall. Substantially all transmission fluid received by the pump and motor mechanism is passed through the filter element.
The present invention further provides a transaxle including at least one casing and the hydrostatic transmission drivingly engaged with an axle mechanism. The filter element extends between the impermeable wall and the casing to define a sealed suction chamber therebetween and any debris entrained in the transmission fluid is blocked from entering the suction chamber by the filter element.